Authorities hold grave fears for a missing nine-year-old boy, after a jet ski accident near Mount Isa in north-west Queensland.

It is believed around 11:00am the jet ski crashed into the edge of Lake Moondarra, striking the boy as he was standing on the bank.

A man, 34, and a six-year-old boy who were riding the jet ski were taken to Mt Isa Hospital with suspected head and spinal injuries and fractures.

Police and the State Emergency Service (SES) are searching for the missing child using a helicopter, ground crews and swift water rescue crews.

The lake is a popular spot for weekend campers and boaties.